A stack refers to multiple units occupying a single tile. Stacking features in earlier Civilization games, spin-offs, and derivative properties, with the exact mechanics varying by title. During combat, a single unit defends on behalf of the stack, selected based on multiple factors within the game rules. Stacked workers combine their output when given the same order, enabling tile improvements to be completed faster.
In Sid Meier's Civilization, Civilization II, and Sid Meier's Colonization, units can be stacked theoretically infinitely. However, loss in combat results in the destruction of the entire defending stack when not stationed in a city or Fortress. Stacks in the field cannot be targeted for espionage.
Sid Meier's Alpha Centauri adopts the framework of the original games, but modifies collateral damage to a percent of defending units' hit points, rather than a guaranteed "stack wipe".
Stacks in Civilization III and the Civilization Revolution games are not subject to collateral damage. Attacks (including bombardment) target only the immediate defender, leaving the rest of the stack unscathed.
Civilization IV and Civilization IV: Colonization combine aspects of both Civ III and Alpha Centauri: siege units and units with the flanking attribute can inflict collateral damage on some or all of the stack; combat is otherwise conducted one-on-one.
Beginning in Civilization V, stacking is replaced with "one unit per tile" (1UPT) as part of a summary redesign of the combat system, with limited stacking permitted between military and civilian units.